Solid oxide fuel cells, otherwise known as ceramic fuel cells, present an environmentally friendly alternative to mainstream electrical energy production processes involving the combustion of fossil fuels. Solid oxide fuel cells enable the catalytic conversion of chemical energy stored in hydrogen into electrical energy without the concomitant release of greenhouse gases. The generation of electrical current by a solid oxide fuel cell using a hydrogen fuel results in the production of water as opposed to the production carbon dioxide, nitrous oxides, and/or sulfur dioxides associated with the combustion of fossil fuels.
In addition to hydrogen, solid oxide fuel cells are operable to function on a wide variety of fuel sources. Fuel sources in addition to hydrogen include hydrocarbons such as methane, natural gas, and diesel fuel. Hydrocarbon fuel sources are reformed into hydrogen for use with solid oxide fuel cells. Hydrocarbon reforming can be administered prior to entry into the fuel electrode or can be administered at the fuel electrode of a solid oxide fuel cell. The ability to function on a wide variety of fuels distinguishes solid oxide fuel cells from other fuel cells which lack the ability to operate on various fuels. Furthermore, the ability of solid oxide fuel cells to administer hydrocarbon feedstock reformation frees such fuel cells from the limitations associated with hydrogen production and distribution.
Currently, solid oxide fuel cells operate at high temperatures ranging from about 800° C. to 1000° C. As a result of high operating temperatures, solid oxide fuel cells require the use of exotic materials which can withstand such operating temperatures. The need for exotic materials greatly increases the costs of solid oxide fuel cells, making their use in certain applications cost-prohibitive. Moreover, high operating temperatures often induce grain growth in the various component layers of a solid oxide fuel cell, including the electrolyte. Grain growth can reduce ion conductivity through the electrolyte thereby degrading the efficiency of the solid oxide fuel cell. High operating temperatures additionally exacerbate stresses caused by differences in coefficients of thermal expansion between component layers of a solid oxide fuel cell. Differences in coefficients of thermal expansion between component layers of a solid oxide fuel cell can lead to cracking at interfaces of the layers as well as within the layers. Differences in coefficients of thermal expansion between the air electrode and the electrolyte, for example, can lead to cracking at the electrode-electrolyte interface and within the electrode and/or electrolyte.
In certain circumstances, a solid oxide fuel cell can operate “in reverse” to electrolyze water into hydrogen gas and oxygen gas by inputting electrical energy. In other circumstances, a solid oxide electrolyzer cell can be designed primarily for use as a hydrolyzer, generating hydrogen and oxygen for later use. In still other circumstances, an electrolyzer cell can be used for other purposes, such as extraction of metal from ore and electroplating. In conventional electrolyzers, electrical energy is lost in the electrolysis reaction driving the diffusion of ions through the electrolyte and across the distance between the electrodes. Also, the ability to conduct electrolysis at higher temperatures would improve the efficiency of the electrolysis. However, at higher temperatures, electrolyzers face similar thermal stresses and cracking caused by differences in coefficients of thermal expansion between component layers of the solid oxide electrolyzer cell. Accordingly, better matching of coefficients of thermal expansion is desired for electrolyzer cells.
A lambda sensor is a device typically placed in the exhaust stream of an internal combustion engine to measure the concentration of oxygen. That measurement allows regulation of the richness or leanness of the fuel/air mixture flowing into the engine. If the fuel/air stream contains too much oxygen, the quantity λ is greater than 1, and the mixture is too lean. If the fuel/air stream contains too little oxygen, then λ<1 and the mixture is too rich. λ equals 1, the ideal situation, when the mixture contains a stoichiometrically equivalent concentration of oxygen and hydrocarbon to allow for complete combustion. A lambda sensor positioned in the exhaust stream detects the amount of oxygen in the combustion products, thereby providing feedback regarding richness or leanness. Lambda sensors and other sensors rely on the diffusion of oxygen anions (O2−) and other ions through barrier materials in ways similar to the manner in which oxygen anions diffuse through a solid electrolyte of a solid oxide fuel cell. Moreover, given the high operating temperature of lambda sensors and similar devices, sensors face thermal stresses, cracking, and delamination issues similar to those facing fuel cells and electrolyzers. Accordingly, embodiments of the present invention provide for improved sensor technology by addressing mismatching of coefficients of thermal expansion, among other reasons.